uapi: add missing install of sync_file.h

As part of the sync framework destaging, the sync_file.h header
was moved, but an entry was not added on Kbuild to install it.
This patch resolves this omission so that "make headers_install"
installs this header.

Fixes: 460bfc41 ("dma-buf/sync_file: de-stage sync_file headers")
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Emilio López <emilio.lopez@collabora.co.uk>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160927143142.8975-1-emilio.lopez@collabora.co.uk